1. Field of Invention
The invention relates to a method for forming metal wires and, in particular, to a method for forming metal wires by microdispensing a pattern on substrate.
2. Related Art
Classical PCB processes include the step of dry metal film attach to substrate, spin photo resist, mask pattern & development, etching photo resist, the second metal plating, and cleaning resist etc. complex processes. For many years, screen-printing and photolithography have been the predominant methods of imaging in PCB manufacturing processes. These techniques have served the industry well and provided the desired image resolution at an acceptable cost. Both methods have their own individual strengths and weaknesses and manufacturers select the most appropriate process for their requirements in primary track imaging, solder mask application and legend printing. These methods suffer from the drawback of being analogue processes and hence require conversion from a digital data file to converted printable image via initial production of a screen or photo tool. This adds cost and time delays to the preparation of boards particularly prototype and short run board production.
As electronic devices become more compact and lighter, the PCB is forced to have smaller hole diameters, higher densities, more layers, and thinner circuit lines. In general, the PCB uses glass fiber cloths or soft materials to form a flat substrate, and then using an adhesive agent or hot press to form a metal layer or copper foil attached to the substrate. The metal layer is then etched to form the desired metal wires. However, halide is added into the ingredients of the adhesives. This is incompatible with the rule that halide are forbidden in all electronic products set by the Europe standard in 2004. Moreover, the adhesives are often added with ionic impurities. This makes the substrate have worse dielectric and insulating properties and be likely to deform at high temperatures. All such facts will reduce the reliability of the substrate. At the same time, because of chemical etching occurred to the adhesives, the metal junction strength gets lower.
Therefore, a metal layer can be formed on the surface of the substrate by deposition and photolithography etching. Electroless plating or sometimes called chemical plating is a method to deposit a metal layer. Without imposing an external voltage, the metal ions in a solution are deposited on the surface of a solid through an autocatalytic chemical reaction. Such a reaction is very similar to electroplating. These materials in the solution have oxidization/reduction reactions on the surface of the solid, the electrons directly transmit on the surface and it is therefore different from electroplating using an external field. To satisfy the thin line requirement on the circuit board, the mask preparation and metal layer etching become harder. Moreover, different types and sizes of metal wires require different masks. This inevitably increases the manufacturing costs.
Using the properties of electroless plating, a method of directly forming a metal pattern on the substrate as the metal wires is developed. As electroless plating has to be performed on an activated or catalyzed surface, one can selectively form a catalytic layer at places for growing metal wires before electroless plating. As disclosed in the U.S. Pat. No. 6,521,285, a selective electroless plating means first forms a print mold with the metal wire pattern. The print mold, coated with a catalytic agent, stamps on the substrate, so that the substrate surface is printed with a catalytic layer for forming metal wires. Finally, a metal layer is grown on the surface of the catalytic agent in an electroless plating means. This method also requires the uses of different print molds for different types and sizes of metal wire catalytic agents. The line width of the circuit is determined by the precision in carving the print mold.